An image sensor is a semiconductor device that converts an optical image into an electrical signal. The image sensor may be classified as a charge coupled device (CCD) and a complementary metal oxide silicon (CMOS) image sensor (CIS).
The CCD is noted in having complicated driving process and excessive power consumption. Because it requires numerous photo steps, the CCD has a complicated manufacturing process. Accordingly, use of the CIS has been provided to overcome such disadvantages of the CCD.
A CIS may include a photodiode and a metal oxide silicon (MOS) transistor in each unit pixel to sequentially detect electrical signals of the respective unit pixels in a switching mode, thereby realizing an image. The CIS may also include a photodiode region and a transistor region. The photodiode region serves to receive an optical signal and convert the optical signal into an electrical signal that is processed by the transistor region. While the CIS may overcome disadvantages of the CCD, it has disadvantages such as the creation of noise due to a dark current. Like leakage current, the dark current tends to rapidly increase in correspondence to increases in temperature.